Various techniques have been known that form a concavo-convex pattern on a surface of an object and improve the function of the object with the concavo-convex pattern. As an example, with electrical steel sheet, a technique is known that forms fine grooves on the surface of electrical steel sheet and reduces the iron loss of the sheet. When processing the surface of an object to improve its function, the shape of a concavo-convex pattern formed on the surface directly affects the quality of the object. Accurate measurement of the shape of the concavo-convex pattern formed on the surface of an object is thus quite important in managing the manufacturing process and assuring product quality.
For example, Japanese Patent Application Laid-open No. 10-89939 describes a surface shape measuring method that successively measures concavo-convex shapes formed on the surface of an object on the production line. That surface shape measuring method measures the amount of displacement between a displacement meter and the object using the displacement meter arranged to relatively move with respect to the object and obtains the cross-sectional shape of the object. The depth (or the height) and the width of the concavo-convex shape are each calculated based on the obtained sectional shape.
As another surface shape measuring method using an optical displacement meter (a laser displacement meter), Japanese Patent Application Laid-open No. 2011-99729 describes a technique that measures the depth and the width of a groove based on a signal of reflection intensity together with a signal of displacement. With the technique described in JP '729, the depth and the width of a groove are measured by excluding an abnormal value of the displacement signal detected at a slant portion of the groove based on the signal of reflection intensity.
However, because the surface shape measuring method described in JP '729 requires a signal of reflection intensity in addition to a signal of displacement as information to be obtained from an optical displacement meter, the method is inapplicable for an optical displacement meter that receives no outputs from a signal of reflection intensity or an optical displacement meter with a function to obtain sufficient reflection intensity by adjusting the intensity of irradiation light and photodetector gain.
Furthermore, when measuring the shape of a fine groove with a triangulation optical displacement meter, the measured value is problematically unstable at a slant portion of the groove due to a shortage of a received light amount. With a displacement meter that adjusts irradiation light and photodetector gain, a shortage of a received light amount less occurs. Instead, such a phenomenon occurs that the shape at a slant portion of the groove is misrecognized, because that kind of displacement meter receives secondary reflection light caused by the reflection of the light applied to the slant portion of the groove in multiple directions inside the groove. Due to such misrecognition, abnormal displacement, in which the groove is recognized deeper than its actual depth, is frequently observed. The surface shape measuring method described in JP '939 thus has a direct problem that the depth of a groove is incorrectly measured.
Furthermore, use of an optical displacement meter to measure the surface of steel sheet may cause comparatively large noise for the size of a groove formed on the surface. The surface shape measuring method described in JP '939 thus has another problem that the position of the groove is incorrectly detected due to the noise caused in the convex direction opposite to the shape of the groove (concave shape).
It could therefore be helpful to provide a surface shape measuring method and a surface shape measuring device that can eliminate disturbance in displacement data and accurately measure the size of a groove formed on a surface of an object using only the displacement data on the surface of the object measured by using a displacement meter.